The present invention relates to a process for the production of Raney nickel catalysts having a long useful life and high product selectivity. This invention also relates to the hydrogenation of organic compounds, particularly the hydrogenation of aromatic nitro compounds prepared in the presence of these Raney nickel catalysts.
The production and use of Raney nickel as a hydrogenation catalyst for aromatic nitro compounds, such as, for example, nitrobenzene, nitrotoluenes, dinitrotoluenes, chlorinated nitro aromatic compounds and the like, is known and has been described. See e.g. R. Schröter, Angew. Chem. 1941, 54, 229 or EP-A-0 223 035. The production of Raney nickel catalysts is usually carried out starting from a pre-alloy comprising aluminium and nickel and optionally one or more further sub-group metals as catalyst precursor. The alloy is obtained, for example, by melting or reactively grinding the starting metals. Raney nickel catalysts can be modified by alloying the starting alloy with other metals in order to improve their activity, selectivity and stability, especially at elevated temperatures. This doping of the catalyst by addition of a very wide variety of metals to the Al—Ni melt of the catalyst precursors is known and described in, for example, DE-A 40 01 484 and DE-A 35 37 247. The catalyst precursors are produced by atomisation of the Al—Ni metal melt, or are cast and then comminuted mechanically. The catalyst is then freed by extracting some or all of the aluminium from the alloy using a base as described in, for example, DE-A 27 13 374. The catalytic activity of the catalysts obtained from the alloys is dependent inter alia on the qualitative and quantitative composition of the alloy, the structure and constitution of the alloy, and accordingly, on the resulting structure and constitution of the catalyst.
The hydrogenation of aromatic nitro compounds is a reaction that is frequently carried out on a large scale. Raney nickel catalysts are often used for this purpose. The catalyst lifetime, product selectivity, structures and constitution of the starting alloys and the solidification rate are scarcely correlated. In the ternary systems of Al—Ni-additional metal in particular, the starting alloy may contain a large number of phases which exhibit no or only poor activities, high catalyst consumption and low product selectivity in the resulting catalyst. DE-A 19 753 501 describes the use of amorphous, partially amorphous or microcrystalline alloys which are produced by rapid solidification, for the production of RaNi catalysts, to increase the catalyst lifetime, and accordingly, to reduce the catalyst consumption. The production processes described therein for the pre-alloy include the pouring out of a metal melt onto a rotating cooling roller or into the gap between two rotating cooling rollers, as well as melt extraction.
According to A. Molnar, G. V. Smith, M. Bartok, Advances in Catalysis, 36, 329-383 (1989), high cooling rates or solidification rates of from 104 to 107 K/s can be achieved by melt spinning or by the production of metal strips. This can be effected, for example, by pressing an alloy melt onto or into the gap between two rotating cooling rollers or by pouring out onto a cooled rotating plate, as well as by processes known to the person skilled in the art as the melt extraction process (i.e. melt extraction rapid solidification technology, MERST), or as the melt overflow process (i.e. melt overflow rapid solidification technology, MORST).
In melt overflow technology, a metal melt flows in a thin layer generally over a horizontal overflow edge onto a rotating cooling roller, with rapidly solidified metal fibers or metal flakes being formed. The overflowing of the melt onto the roller is made possible, for example, by tipping the melting crucible, but can also be carried out by displacing the melt by means of, for example, a plunger that dips into the melting crucible. By shrinkage of the alloy as it cools on the cold metal surface, and as a result of the centrifugal force of the rotating roller, the flakes or fibers are thrown from the surface thereof. The melt overflow process can be carried out in air, under an inert gas, or alternatively, in a vacuum chamber.
The technology for the production of the pre-alloy by means of the melt overflow process is described inter alia in U.S. Pat. No. 5,170,837 and also in U.S. Pat. No. 4,907,641.
In melt extraction, a rotating cooling roller provided above the melting crucible comes into contact with the surface of the metal melt and, as a result of the rotation, draws rapidly solidified metal fibers from the melt. By shrinkage of the alloy as it cools on the cold metal surface, and as a result of the centrifugal force of the rotating roller, the flakes or fibers are thrown from the surface thereof.
Melt extraction can be also be carried out in air, under an inert gas, or alternatively, in a vacuum chamber.
The technology for the production of the pre-alloy by means of the melt extraction process is described inter alia in O. Andersen, G. Stephani, Metal Powder Report, 54, 30-34 (1999).
A further method of rapid solidification comprises pouring a metal melt onto a rotating cooling plate, and allowing the rapidly solidified alloy to be thrown tangentially from the plate.
Pouring onto the cooling plate can be carried out in air, under an inert gas, or alternatively, in a vacuum chamber.
In melt overflow technology, melt extraction technology and in the case of pouring out onto rotating rollers or a rotating plate, it is possible, according to A. Molnar, G. V. Smith, M. Bartok, Advances in Catalysis, 36, 329-383 (1989), to achieve cooling rates which are very much greater than 104 K/s. In contrast to conventional rapid solidification processes, such as the atomisation of a metal melt in water, as described, for example, in EP-A-0 437 788, the formation of an undesirable oxide content is largely suppressed in such processes.
In the case of the pouring of a metal melt onto or into the gap between two rotating cooling rollers, in the melt overflow process, in the melt extraction process or in the pouring of a metal melt onto a rotating cooling plate, endless fibres or endless strips are usually produced in the production of ductile metal alloys. The cooling rollers or cooling plates used in the production of such endless fibers or endless strips have surfaces that exhibit no structuring transversely to the direction of rotation. The entire surface of the cooling roller or cooling plate is used for cooling and solidifying the melt, and the cooling capacity is accordingly fully utilised. As a result, it is possible to achieve a very uniform, high solidification speed and the formation of a very homogeneous microcrystallinity. The use of cooling rollers and cooling plates without structuring transversely to the direction of rotation is also advantageous especially because such structures on the surface of the cooling rollers or cooling plates can have an adverse effect on the microstructure of the alloys produced, because the cooling speed at the beginning and at the end of such surface structures on the cooling roller or cooling plate is lower than in the middle. Furthermore, ductile metal alloys are advantageously produced in the form of endless fibers or strips because the formation of agglomerates which have not been rapidly solidified can be suppressed to the greatest possible extent owing to the uniform solidification rate of the alloy on the cooling roller or cooling plate.
However, Raney nickel pre-alloys having a high aluminium content are not ductile, but brittle materials. Therefore, the rapid solidification of RaNi pre-alloys using rotating cooling rollers or cooling plates by, for example, the melt overflow process or the melt extraction process, in accordance with the prior art yields long fragments of fibers or strips of irregular length. These fibers and strips of undefined length tend to interlock and mat in the product container, during conveying and during transportation, are not pourable and have a decidedly low bulk density during filling and transportation.
It is therefore necessary to bring the fibers or strips of the pre-alloy to a processable and transportable size suitable for conveying, transportation and further processing, by means of a grinding process provided downstream. This additional process step requires an additional outlay in terms of apparatus and energy in the production of the pre-alloy. Moreover, the additional energy supplied during grinding alters the metal structure of the pre-alloy and its microstructure. This mechanically induced recrystallisation is described, for example, in J. Friedrich, U. Herr, K. Samwer, Journal of Applied Physics, 87, 2464 (2000). If grinding of the pre-alloy is omitted, however, a considerably greater transport volume is required, and conveying of the interlocked and matted alloy, for example, by means of a conveyor belt, a conveyor screw or an air stream, is made considerably more difficult or impossible.
There is therefore a need for a simple and economic process for the production of rapidly solidified RaNi catalysts which are suitable particularly as catalysts for the hydrogenation of nitro aromatic compounds to the corresponding amines wherein the catalysts having long useful lives and high selectivity.
Accordingly, the object of the present invention was to provide Raney nickel catalysts, and a simple and economic process for their production, in which the Raney nickel pre-alloys produced by rapid solidification do not interlock or mat and can be transported and treated further, without additional outlay.
It has now been found that the particle size or fiber size of Raney nickel pre-alloys can be markedly reduced compared with the alloys conventionally obtained by rapid solidification, if cooling rollers structured by means of transverse grooves are used for the pouring out onto or into the gap between two cooling rollers, for the melt extraction or for the melt overflow process, or if cooling plates structured by means of grooves extending outwards from the axis of rotation are used for the pouring out onto a rotating cooling plate. The short fibers or short strips produced according to the invention have a markedly higher bulk density, are pourable, do not tend to mat, and can be transported without difficulty by means of conventional conveyor devices such as, for example, conveyor belts, conveyor screws. Surprisingly, the catalytic properties of the Raney nickel catalysts produced in accordance with the present invention even exceed the catalytic properties of the Raney nickel catalysts produced in accordance with the prior art by rapid solidification on cooling rollers or cooling plates without surface structuring transversely to the direction of rotation. This is seen, for example, in the hydrogenation of dinitrotoluene, in an increased yield of toluylenediamine from dinitrotoluene, and an increased lifetime of the catalyst.